Why Autoclaves Fail Spore Tests (and How to Prevent It)

By MS Dental Works · City of Industry, CA · Updated 2026

A positive spore test is one of the most stressful things a dental office can find on a Monday morning. It means the biological indicator you ran came back showing live, growing spores — and that the load that cycle was supposed to sterilize may not have been sterile at all. This guide explains what a failed spore test actually tells you, the common operator and mechanical causes behind it, and how to keep it from happening again. It applies to most steam chamber sterilizers (Midmark, Tuttnauer, Statim, and similar); always follow your manufacturer's manual and your CDC reprocessing protocol where they differ.

If your autoclave just failed a spore test, treat it as out of service now. Standard CDC guidance is to remove the unit from use, identify and correct the cause, and re-test with negative results before returning it to service. See our autoclave failing a spore test page for the immediate steps.

What a positive spore test really means

A biological indicator (BI) contains highly resistant bacterial spores. If those spores are killed, your sterilizer is delivering the temperature, pressure, and steam contact needed to reliably kill everything else. If they survive — a positive result — sterilization conditions were not achieved somewhere in that cycle, even if the gauge and the cycle printout looked normal.

That last point trips up a lot of offices. A cycle can hit its time and pressure setpoints and still fail to sterilize if the temperature ran a few degrees low, steam couldn't reach every surface, or a sensor was misreading. The spore test is the only monitor that confirms the biological result, which is exactly why the CDC recommends it. A single chemical indicator changing color is not the same thing.

Operator and load causes (check these first)

Before assuming the machine is broken, rule out the things that cause the majority of failures and cost nothing to fix:

Overloading the chamber. Packed-in pouches and stacked cassettes block steam from circulating. Steam must contact every surface — crowding is one of the top causes of failed BIs.
Improper packaging. Pouches sealed wrong, wrapped too tightly, or facing the wrong way trap air and prevent steam penetration.
Wrong cycle selected. Running a wrapped or pouched load on an unwrapped/flash setting can shorten exposure below what those items need.
BI placed in the wrong spot. The indicator should sit in the hardest-to-sterilize location per your protocol, but a process error can also produce a false positive — confirm against your control.
Water quality. Tap or hard water leaves scale and can interfere with steam quality. Use distilled or steam-process water only.

Mechanical causes a technician looks for

If technique and loading are correct and the failure repeats, the problem is usually inside the unit. These are the common mechanical culprits — actual diagnosis requires testing the specific machine against its manual:

Worn or hardened door gasket. A cracked, flattened, or aging gasket lets steam escape, so the chamber never holds true sterilizing conditions. This is one of the most frequent mechanical causes of a failed spore test.
Failing heating element. A degrading element can let pressure look right while the actual temperature falls short of the target — exactly the gap a spore test catches.
Inaccurate temperature or pressure sensor. A drifting or faulty sensor makes the cycle "think" it reached spec when it didn't. Sensors should be verified, and on many units calibration or replacement is a technician task.
Scale and mineral buildup. Deposits from non-distilled water clog valves, foul sensors, and disrupt steam generation over time.
Steam trap, valve, or chamber-seal issues. Anything that lets the chamber lose pressure or fail to purge air properly can prevent full steam contact.

Don't guess at fault codes. Error codes vary by make and model and shouldn't be interpreted from memory. If your unit is showing a code, our free troubleshooter can give you a preliminary read, and a technician will confirm it against the manufacturer's service documentation.

What to do after a failed spore test

A practical, CDC-aligned sequence — verify the exact recall and documentation steps against your office protocol and current state board requirements:

Take the sterilizer out of service immediately and label it clearly.
Repeat the BI test (with a fresh control) to rule out a process error or a bad indicator.
If it fails again, do not return the unit to use — quarantine and review the instruments processed since the last negative test, per your recall protocol.
Document the failure, the corrective action, and the negative re-test results.
Have the unit serviced; return it to use only after it passes biological testing again.

How to prevent spore-test failures

Use distilled or steam-process water only and keep the reservoir clean.
Load loosely — let steam circulate, and never pack the chamber.
Package and orient instruments per the manufacturer's instructions.
Select the correct cycle for each load type, every time.
Inspect the door gasket weekly and replace it when worn.
Keep up with daily, weekly, and monthly preventive maintenance.
Run and log your weekly spore test — and test every load with an implantable device.

For the full routine, see our autoclave preventive maintenance checklist. Consistent technique plus PM prevents the large majority of failures — and catches the rest before they become a recall.

Autoclave failing spore tests?

MS Dental Works repairs dental autoclaves across LA County — same-day dispatch, loaner units, and a tech who arrives knowing the likely fix. We'll test the gasket, heating element, and sensors and get you back to passing. No travel fee within 30 miles.

Request service → Call 626-422-2315

Frequently asked questions

A positive (failed) biological indicator means the spores survived the cycle, so the load was not reliably sterilized. The autoclave should be removed from service, the cause identified and corrected, and the unit re-tested with negative results before it returns to use. Follow your CDC reprocessing guidance and state board requirements.

No. Standard CDC guidance is to take the sterilizer out of service after a positive spore test until the problem is found and fixed and the unit passes repeat testing. Instruments processed since the last negative test may need to be recalled and reprocessed — check your office protocol and local requirements.

A cycle can reach its time and pressure targets on the gauge yet still fail to sterilize if temperature is slightly low, steam contact is blocked by overloading or poor packaging, or a sensor is reading inaccurately. That is exactly why biological monitoring exists — it catches failures that the cycle readout cannot.

The most common causes are operator factors (overloading, improper packaging, wrong cycle) and mechanical issues (a worn door gasket leaking steam, a failing heating element, an inaccurate temperature or pressure sensor, or scale from tap water). A technician can isolate which one is at fault.

Use distilled or steam-process water, don't overload the chamber, package and space instruments correctly, run the right cycle for the load, inspect the gasket weekly, and keep up with preventive maintenance. Consistent technique plus PM prevents most failures.